In large-sized aircraft such as airplanes for transporting tens or hundreds of passengers, the supply of electrical power becomes a very significant element in the general design of the aircraft. The reason for this is that the amount of electrical equipment installed on board and used either for the operation of the aircraft or for the onboard services is growing and is consuming more and more power.
This energy is produced by alternators coupled to the engines of the aircraft and the alternators usually supply a three-phase voltage of 115 volts r.m.s. between neutral and phase, at a frequency of 400 Hz. This voltage is carried inside the aircraft by electrical cables whose cross-section is proportional to the square of the value of the current that these cables must be able to carry. Typically, several hundred meters of cable capable of carrying several kilowatts are needed. This results in a very large weight of copper or aluminum to be installed into the aircraft.
In consequence, it was seen that it could be preferable to now design aircraft in which the power is carried at a minimum of 230 volts, in order to divide substantially by 4 the cross-section of the cables carrying the power. The alternators of such aircraft will therefore be designed to directly supply a three-phase voltage at 400 Hz and 230 volts r.m.s. between phase and neutral. In addition, these modern aircraft will now be equipped with a distribution network of DC electrical power, typically at 540 volts (plus or minus 270 volts with respect to the metallic structure of the aircraft). The advantage of the DC power distribution is to allow, by means of variable-frequency inverters, an individual speed control to be effected for certain synchronous or asynchronous motors present in the aircraft (compressors, air-conditioning units, fuel pumps, etc.).
Furthermore, aircraft have to consume electrical power when they are stationary on the ground at an airport with engines stopped. This power is required in order to provide lighting, air-conditioning, maintenance, starting, etc. functions.
They are therefore connected, via a three-phase connector accessible on the exterior of the aircraft, to electrical power generator units situated on the ground, administered by airports. The generator units all supply three-phase power at 115 volts r.m.s. since the majority of airplanes are equipped for operation with 115 volts r.m.s. It may be envisioned that, in the future, airports equip themselves with generators supplying both 115 volts and 230 volts, or that special units supplying 230 volts be provided for the case where an airplane equipped with 230 volt systems might land. However, that implies a cost that the airports do not wish to carry and this solution is only to be envisioned in the very long term when the number of aircraft equipped with 230 volt systems will be very significant.
In the immediate term, the solution is to provide a three-phase transformer on the aircraft placed between an external power supply connector (designed to be hooked to the ground generator) and the 230 volt power supply network on the airplane. This transformer adds more weight and takes up additional space solely for this reason of airport logistics.
The present invention proposes a solution for limiting the drawbacks resulting from this situation. This solution is applicable when the aircraft possesses a DC electrical power distribution network receiving the power from a three-phase alternator and an AC-DC converter (in practice an autotransformer followed by a rectifier bridge) in order to transform the AC power into DC power.